Cygwin: pty: add pseudo console support.

- Support pseudo console in PTY. Pseudo console is a new feature
  in Windows 10 1809, which provides console APIs on virtual
  terminal. With this patch, native console applications can work
  in PTYs such as mintty, ssh, gnu screen or tmux.

winsup/cygwin/fork.cc

@@ -134,6 +134,30 @@ child_info::prefork (bool detached)
 int __stdcall
 frok::child (volatile char * volatile here)
 {
+  cygheap_fdenum cfd (false);
+  while (cfd.next () >= 0)
+    if (cfd->get_major () == DEV_PTYM_MAJOR)
+      {
+	fhandler_base *fh = cfd;
+	fhandler_pty_master *ptym = (fhandler_pty_master *) fh;
+	if (ptym->getPseudoConsole () &&
+	    !fhandler_console::get_console_process_id (
+				ptym->getHelperProcessId (), true))
+
+	  {
+	    debug_printf ("found a PTY master %d: helper_PID=%d",
+			  ptym->get_minor (), ptym->getHelperProcessId ());
+	    if (ptym->attach_pcon_in_fork ())
+	      {
+		FreeConsole ();
+		if (!AttachConsole (ptym->getHelperProcessId ()))
+		  /* Error */;
+		else
+		  break;
+	      }
+	  }
+      }
+
   HANDLE& hParent = ch.parent;
 
   sync_with_parent ("after longjmp", true);